This invention relates to guitar amplifiers and, more specifically, to a vacuum tube guitar amplifier to provide presence control.
Presence controls for vacuum tube guitar amplifiers are well known and have been in use for many decades. Unlike preamplifier tone controls, the traditional presence control operates in the negative feedback loop of the power amplifier and phase splitter circuitry. Typically, the presence control allows player adjustment of a higher frequency range than a typical treble tone control and operates by attenuating to ground a portion of the negative feedback spectrum. Using a variable resistor as the control element, whatever portion of the negative feedback signal is deleted results in a corresponding increase in circuit gain and audible “brightness”. In some traditional circuits, the fixed resistance value across the presence potentiometer may also be important as part of the driver circuit, in that case making maintenance of this fixed or maximum resistance necessary to insure proper circuit performance.
Some prior art guitar amplifiers include switchable performance parameters, including the ability to utilize or eliminate negative feedback. The sound of an amplifier with its feedback removed is characteristically very brash and aggressive, this sound having become extremely popular in current guitar music. In contrast to the so-called traditional “vintage” setting, which uses ample negative feedback and allows for a presence control, the non-negative feedback characteristic could be called “modern” sounding and is the tonal foundation for much of the heavy-metal, thrash and grunge styles of popular guitar music. Unfortunately however, the presence control is no longer functional in such “modern” circuits because the negative feedback on which it operates has been de-activated.
Over the years, a long felt need has arisen in that such so-called modern voicings (without negative feedback) might be overly bright and aggressive sounding, thus inviting some user-adjustable high-frequency attenuation. However, it is under precisely these conditions that the traditional presence control is no longer capable of functioning Even though traditional negative feedback style presence controls are only capable of increasing brightness (by attenuating high frequencies from the negative feedback), there is now the need for an adjustable control which does just the opposite and attenuates brightness. Further, confusion arises in the mind of the player when the knob on the amplifier control panel labeled “presence” becomes non-functional when modern performance is selected due to the absence of the negative feedback.
The obvious solution would appear to be merely a reduction of the negative feedback (rather than eliminating it entirely) which would leave the presence control at least partially functional. Unfortunately, this approach is, at best, a weak compromise because any residual negative feedback greatly diminishes the searing brashness of the modern sonic character while still not providing an audibly effective presence control. Thus, merely reducing negative feedback achieves neither the desired modern sonic amplifier characteristic nor does it provide for satisfactory presence control operation.